This application is based on Japanese Priority Application Hei 8-56944, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates a batch-type kiln having particular applicability in the performance of firing treatments, such as a dewaxing step and a firing step, on a ceramic product, such as a ceramic capacitor.
2. Description of the Related Art
Conventional batch-type kilns comprise a heating chamber and a table. The heating chamber is formed by placing a rod-shaped or U-shaped heater within a box-shaped or cylindrical kiln body. A table is disposed at the bottom portion of the heating chamber and is capable of being raised and lowered. A sagger containing a ceramic product is placed on the table, and the table is raised to place the sagger into the heating chamber for firing treatment. Upon completion of the treatment, the table is lowered and the sagger is removed from the kiln.
Conventionally, dewaxing and firing steps are successively performed on the ceramic product. However, in so treating the ceramic product in conventional batch-type kilns, the treatment of an organic binder produced during dewaxing becomes a problem. Decomposition of the binder produces a gas which is heavier than air. This gas flows downward towards the bottom of the heating chamber. There is a narrow gap between an outer peripheral portion of the table and the kiln body, the temperature of the gap being less than 100xc2x0 C. during firing. As a result of this relatively low temperature in the gap, the gas produced from binder decomposition which flows therethrough liquefies or solidifies, accumulating as tar which produces an offensive smell. In addition, the kiln wall and the table are made of a felt-like heat-insulating material. The gas produced by binder decomposition penetrates into this heat-insulating material, causing the penetrated binder component to carburize due to thermal decomposition or the like. This effect can result in reduced heat insulation, electrical leakage, or other problems.
Accordingly, an object of the present invention is to provide a batch-type kiln which can prevent accumulation of a binder component in, for example, a gap formed between a table and a kiln wall.
According to one aspect of the invention, there is provided a batch-type kiln. The kiln comprises a kiln body and a heating chamber disposed within the kiln body, which has a heater disposed therein. A table is disposed at the bottom of the heating chamber, the table having a peripheral portion and an upper surface for supporting an object to be treated. The peripheral portion of the table and a portion of the kiln body define a gap therebetween. This gap forms a gas-introducing path for introducing a gas into the heating chamber.
According to a second aspect of the invention, there is provided a batch-type kiln, which comprises a kiln body and a heating chamber disposed within the kiln body, the chamber having a heater disposed therein. A table is disposed at the bottom of the heating chamber for placing thereon an object to be fired. The table is capable of being raised and lowered. A gas-introducing path is also provided, for introducing into the heating chamber a gas which is the same type as an atmospheric gas in the kiln from a gap between an outer peripheral portion of the table and the kiln body.
The gas produced in the heating chamber by binder decomposition flows downward, attempting to flow into the gap between the table and the kiln body. However, by supplying the same type of gas as the atmospheric gas in the kiln through the gap and into the heating chamber prevents the gas formed by binder decomposition from flowing into the gap, thus preventing accumulation of the binder component in the gap. In addition, the gas produced by binder decomposition in the vicinity of the kiln body becomes agitated due to the inflowing gas, thereby reducing binder component adhesion onto and penetration into the kiln body and the table.
The gas supplied into the heating chamber is the same type as the atmospheric gas in the kiln, so that the atmosphere in the kiln will not be disturbed. In the batchtype kiln in which a degreasing step and a firing step are is successively performed, the aforementioned gas is introduced only during the dewaxing step, such that the gas is not supplied during the firing step.
In the above-described kiln, it is preferable to supply a gas which is of the same type as the atmospheric gas, along the horizontal face of a step (formed at the outer peripheral portion of the table) from the outer periphery inwardly, and then to allow the gas to flow upwardly along the vertical face of the step and into the heating chamber.
In such a case, the gas-supplying path is non-linear, which reduces the amount of heat escaping from the heating chamber, and allows heat to be left in the gas flowing through this path. As a result, a temperature drop in the kiln occurs less often.
When the outer periphery of the heater is covered with a thermal resistant insulating tube, it is fully protected from the binder component produced during the dewaxing step, thereby eliminating the problem of electrical leakage occurring between heaters or between a heater and the kiln body. In addition, in such a case, effects on the properties of the object to be fired due to evaporation of impurities contained in the heater are reduced. Further, effects on the heater due to evaporation of impurities contained in the object to be fired are reduced.
It is preferable to form a cylindrical air chamber at an outside wall of the kiln, and provide the chamber with an opening for admitting air into the air chamber and an opening for discharging air therefrom. In such a case, when the temperature rises in the kiln, the air chamber is used as a heat-insulating layer. During the cooling process, the accumulated heat in the air layer is removed by allowing the air to circulate via the air admission opening and the air discharge opening, thereby removing the accumulated heat in the air layer for effective cooling.